Eradicating Citrus Greening By Dual Action Targeting of Liberibacter asiaticus and Diaphorina citri

Abstract

Huanglongbing (citrus greening) is caused by the unculturable, gram negative bacterium Liberibacter asiaticus and transmitted by the Asian Citrus Psyllid (Diaphorina citri). Prior research demonstrated that the rifampicin-derivative TPR-1 as well as Palisades Therapeutic (PT) compounds PT159 and PT160 were effective at inhibiting growth of the L. cresens, the only culturable model for Las, showing 100% inhibition at 0.5 μg mL⁻¹, and 80% inhibition at 0.05 μg mL⁻¹.

Research with the PT glucocorticoid antagonist compounds demonstrated an inhibition of bacterial growth in L crescens as well, possibly a synergistic addition to TPR-1. A search of the L. asiaticus genome revealed the presence of 3 putative glucocorticoid response elements (GRE). Putative GRE are present in all organisms and may be involved in organism signaling as well as host-pathogen crosstalk.

In Drosophila, the estrogen-related receptor (ERR) transcription factor is a master regulator of larval and pupal maturation. ERR binds to GRE promoter elements in the genome to regulate the transcription of pathways involved in glucose metabolism. Null mutations of ERR fail to leave pupation and demonstrate elevated glucose levels when compared to wild-type. Treatment of Drosophila larvae with PT compounds demonstrated developmental delay disruptions in pupation, including premature pupation and increased lethality. Pupae that had been fed PT compounds as larvae showed elevated glucose levels when compared to controls. Sequences identified in Drosophila ERR showed high homology to 2 proteins in D. citri (ERR-like 1 and ERR-like 2), leading us to hypothesize that a glucocorticoid antagonist may disrupt psyllid development. Because these genes and signaling pathways are highly conserved between these taxa, we propose that Drosophila serves as a good model for screening potential compounds for the control of psyllid populations.

Collectively, we hypothesize that the dual action of these compounds offers a solution to HLB that can salvage the dying citrus industry in Florida and provide the first effective treatment. By simultaneous targeting of both Las and D. citri, it may be possible to both cure infected trees and block psyllid development in adults feeding on those same trees. Disruption of the psyllid’s ability to carry Las or cure Las from the psyllid gut are also both substantial possibilities.